In recent years, regulation on environmental load substances, such as sulfur components, contained in liquid fuels, such as gasoline and gas oil, has become rapidly severe. Therefore, the production of environment-friendly clean liquid fuels in which the content of sulfur components and aromatic hydrocarbons is low has become essential. One example of a method for producing such clean fuels includes the so-called Fischer-Tropsch synthesis method (hereinafter sometimes referred to as an “FT synthesis method”) in which carbon monoxide is reduced by molecular hydrogen (hydrogen gas). By the FT synthesis method, liquid fuel base stocks that are rich in paraffin hydrocarbons and contain no sulfur components can be produced, and wax can also be produced. This wax can be converted to middle distillate (fuel base stocks, such as kerosene and gas oil) by hydrocracking.
A Fischer-Tropsch synthesis catalyst (hereinafter sometimes referred to as an “FT synthesis catalyst”) that is a catalyst used in a Fischer-Tropsch synthesis reaction (hereinafter sometimes referred to as an “FT synthesis reaction”) is generally a catalyst in which an active metal, such as iron, cobalt, or ruthenium, is supported on a carrier, such as silica or alumina (for example, see Patent Literature 1). In addition, it is reported that in an FT synthesis catalyst, catalyst performance is improved by using a second metal, in addition to the above active metal (for example, see Patent Literature 2). Examples of the second metal include sodium, magnesium, lithium, zirconium, and hafnium, and they are appropriately used according to a purpose, such as an improvement in the conversion of carbon monoxide, or an increase in chain growth probability that can be an indicator of a wax production amount. In the actual use of an FT synthesis catalyst, the combined use of the above second metal is considered, also in terms of keeping a decrease in the activity of the catalyst to a minimum during the FT synthesis reaction.
Examples of performance required for a practical FT synthesis catalyst mainly include catalytic activity, product selectivity, and catalyst life. For factors causing catalyst deterioration that shorten catalyst life among these, there are many examples of studies, such as deposition of a carbonaceous matter during reaction (for example, see Non Patent Literature 1), oxidation of the active metal (for example, see Non Patent Literature 2), and generation of a composite oxide by a reaction between an active metal and a carrier (for example, see Non Patent Literature 3). On the other hand, it is still difficult to recover the activity of a once deteriorated catalyst itself, and the present situation is that activity decrease must be compensated for by additionally introducing a new catalyst into a reaction apparatus. In this method, not only cost increases because the additional introduction of an expensive catalyst is necessary, but also the fact that the amount of the spent catalyst that eventually becomes waste increases has become a problem.